Members of the class of ligands made by condensation of a diamine with two molar equivalents of optionally functionalized salicylaldehyde molecules are commonly referred to as salen ligands or “salens” after the simplest member of the class, N,N′-ethylenebis(salicylimine). The salen ligand may be either symmetric (containing two identical optionally functionalized salicylaldehyde moieties) or non-symmetric (containing two different optionally functionalized salicylaldehyde moieties).
This class of ligands may be generalized further to include compounds formed by condensation of a diamine with two molar equivalents of optionally functionalized β-hydroxy carbonyl compounds. As noted earlier, the ligands may be either symmetric or non-symmetric. These tetradentate ligands form Co(III) and Cr(III) complexes when the two oxygen atoms and the two nitrogen atoms of the ligand form bonds with metal atom. The remaining +1 charge on the metal is balanced by an anionic counterion. Cr(III) and Co(III) salen complexes have been shown to catalyze epoxide hydrolysis, epoxide-CO2 copolymerization, and phenol oxidation, among other reactions. In some cases, isolation of the desired salen complexes comprising monoanionic counterions (nitrate, chloride, acetate, trifluoroacetate, and other monocarboxylic acids for example) is difficult owing to poor precipitation. Often, when attempts are made to precipitate such compounds, a viscous oil or sticky solid that cannot be easily handled until exhaustively dried is formed. This is not an efficient or economical method for isolation on manufacturing scale; furthermore, isolation in this manner does not provide any means for purification, which is desirable for a catalytic species. Alternatively, it would be desirable to precipitate a Cr(III) or Co(III) salen complex directly from a crude reaction mixture by the addition of a co-solvent or the removal of a solvent which dissolves the complex.